Sometimes a person cannot clearly see objects close up or far away. To correct t

Question

Sometimes a person cannot clearly see objects close up or far away. To correct this type of vision, bifocals are often used. The top half of the lens is used to view distant objects and the bottom half of the lens is used to view objects close to the eye. A person can clearly see objects only if they are located between 38 cm and 176 cm away from her eyes. Bifocal lenses are used to correct her vision.

What focal length lens (in cm) should be used in the bottom half of the lens to allow her to clearly see objects 25 cm away?

What focal length lens (in cm) should be used in the top half of the lens to allow her to clearly see distant objects?

Explanation / Answer

For bottom half

used to see near objects. So, v(image distance)=-38cm. u=25cm

Apply thin lens formula

1/f =1/v + 1/u

1/f = -1/38 + 1/25 ;

f=73.08cm

for the top

Now, v=-176cm ; u=infinity

1/f =1/v + 1/u;

1/f = -1/176 + 0

f=176cm

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